1. Field of the Invention
The invention is directed to snubbers (also known as shock arrestors) and methods of controlling the motion of objects with snubbers. In particular, the invention is directed toward hydraulic snubbers and methods of controlling the motion of objects with hydraulic snubbers.
2. Background of the Invention
Hydraulic snubbers or shock arrestors are often used in piping systems to allow slow movement of the pipes due to thermal expansion while protecting the piping systems and equipment from accidental damage arising from abnormal loading or movement conditions due to a shock force or vibration such as experienced during a seismic disturbance. Snubbers are often used in power plants to restrain pipes during seismic conditions since using fixed coupling could cause damage to the pipes during normal events.
FIG. 1 shows a basic schematic of a prior art snubber 100. Snubber 100 would be coupled at one end to a fixed object, such as a power plant wall or floor, while the other end would be coupled to a pipe. A basic hydraulic snubber consists of a cylinder (which includes a body, piston and piston rod or rods), a control valve, and a hydraulic fluid reservoir. Under normal circumstances, such as when the pipe is heating up, piston rod 105 would push piston 110 into body 115. Body 115 is filled with a slightly compressible fluid. As piston 110 extends into body 115, the fluid inside chamber 125 passes through control valve 130 and is displaced into chamber 120. The excess fluid (since the snubber only has one piston rod) is displaced into reservoir 135. On the other hand, as the pipe cools, piston rod 105 exits body 115 causing piston 110 to move within body 115. In that circumstance, chamber 125 will fill as chamber 120 empties. Since chamber 125 has a larger area than chamber 120 more fluid is required to fill chamber 125 than is supplied by chamber 120. This fluid comes from reservoir 135. Under operating circumstances, snubber 100 will function almost invisibly.
Under predetermined conditions, for instance during seismic events, the snubber must be able to lock up to prevent damage to the system. During such conditions, piston 110 may move quickly closing control valve 130. In this case fluid is supplied from reservoir 135. However, as can be seen in FIG. 2, valve 230 and reservoir 235 greatly increases the size of snubber 200, thereby making the space needed to install snubber 200 greater. To minimize the size of the snubber a double ended cylinder (a piston rod on each end of the piston) is utilized with the control valves installed in the piston. An internal reservoir is utilized for fluid expansion and contraction. Unlike a single ended cylinder (one piston rod), fluid is not required to enter or exit the reservoir during normal stroking. During the normal (valves unlocked) mode fluid simply transfers from one side of the piston to the other thru the normally open lockup valves. During modes where one of the valves is closed the fluid on one side of the cylinder is compressed and the other side needs fluid to compensate for the resultant fluid compression. Since the valves are in the piston and not directly connected to the reservoir, the reservoir cannot supply the required fluid. If additional fluid is not provided to fill the vacuum, the vacuum will initially remove any entrained or entrapped air contained in the fluid, creating bubbles. If a sufficient vacuum level is attained, the fluid may vaporize causing even more bubbles. Bubbles will result in a lower spring rate and possible erratic lock-up rates, in order to minimize these effects, some manufacturers of snubbers de-gas the fluid before inserting it into the snubber, however this is a very costly operation. Thus it is desirable to have a snubber without external valves and fluid reservoirs and without the need to de-gas the fluid.